For example, at base stations for mobile phones and transmitter stations for digital broadcast, a highly accurate reference frequency signal that is required to synchronize timings of transmitting signals and frequencies has been supplied by using a reference frequency generating device. Further, in such kind of reference frequency generating device, there exists a reference frequency generating device for controlling a voltage controlled oscillator so as to synchronize a signal with a highly accurate reference signal obtained from, for example, a GPS receiver, and the device outputs the reference frequency signal.
The voltage controlled oscillator generates a frequency that differs according to a controlled voltage inputted thereto, and an example thereof includes a voltage controlled oscillator that uses a crystal oscillator as a resonator. A controlled-voltage to output-frequency characteristic (hereinafter, referred to as “the F-V characteristic”) of this type of voltage controlled oscillator slightly changes with time and according to a change in temperature thereof being used.
Further, the GPS receiver may not receive signals from GPS satellites due to various reasons, such as positions of the GPS satellites, impedance, and an interfering wave, and the reference frequency generating device as described above may not be able to generate the reference signal. Thus, a reference frequency generating device has been proposed, which has a holdover control capability of continuously outputting the highly accurate reference frequency signal even when the reference signal cannot be acquired.
In a situation where the reference signal can be acquired, such a reference frequency generating device can store data used in controlling the voltage controlled oscillator. Further, when the reference signal cannot be acquired, based on the stored data from the past, the voltage controlled oscillator is controlled by a holdover control capability and, thus, the highly accurate reference frequency signal can be outputted over a long time period. For example, Patent Document 1 discloses such kind of reference frequency generating device.
Patent Document 1 discloses a reference frequency generating device for generating a voltage controlled signal for the holdover while considering that a F-V characteristic changes according to a change of a crystal oscillator in temperature (frequency-temperature characteristic).